1. Technical Field
The present invention relates in general to an improved flexible keel joint, and in particular to an improved riser to sleeve interface for flexible keel joints.
2. Description of the Prior Art
Deep water oil operations from floating vessels typically utilize risers or tendons that extend from the vessel to the sea floor. Such floating vessels include tension buoyant towers, compliant towers, and spars in which the structures extend well below the sea surface and are subjected to heave, pitch, and roll motion at the surface. The risers and tendons are connected to the sea floor and pass through openings in the keel or bottom portion of the vessels. The openings in the vessels constrain the pipe forming the risers or tendons when the vessel is moved laterally with respect to the sea floor connection. Such lateral movement produces bending of the pipe at the constraint opening, or rotation of the pipe about the contact of the pipe with the edges of the opening. Bending of the pipe, which is normally under tension, results in fatigue and wear at the constraint opening.
Prior art solutions for accommodating such riser/tendon stress include the use of thick-walled pipes with tapered ends. These thick, tapered wall sections are usually machined from heavy forgings and are very expensive. Another solution utilizes a sleeve member centralized within the vessel opening and a mud line or sea floor connection to receive the lower end of the pipe. The pipe is centralized within the sleeve but otherwise unattached to the sleeve. A third solution uses a centralizing, ring-like device, such as a ball joint, located between the side walls of the vessel opening and the pipe.
In U.S. Pat. No. 5,683,205, a joint is used on a pipe that passes through the vessel opening and is connected to the sea floor. The pipe is centralized within an outer sleeve with large elastomeric rings located at each axial end of the sleeve. Although the rings distribute bending stresses from the pipe to the sleeve by the pipe, they offer very limited axial load support and are subject to wear. An improved design for flexible keel joints is needed.
The invention relates to riser pipe extending from a subsea well to a surface platform. A sleeve is positioned around the riser pipe where the pipe penetrates the keel of the platform. The riser to sleeve attachment of the invention provides a high load carrying capacity in both the axial and lateral direction (or reduced capacity in one of these directions) but permits flexibility for angular offsets between an outer sleeve and a riser pipe.
A conical shaped ring embodiment of the invention is based on providing an attachment between a small diameter ring that engages a riser pipe and a large diameter ring that engages an outer sleeve, wherein the attachment has members that are very flexible in bending but relatively stiff and strong in axial load. The specific configuration of the attachment then determines the direction in which the high load transfer capability exists. The configuration of the conical embodiment acts to allow angular movement of the inner small diameter ring relative to the outer large diameter ring by producing only bending loads in the individual beams. The configuration of the cone, in regard to the angle of the beam relative to the axial direction, determines the relative stiffness and load capacity of this configuration.
In a second or flat ring embodiment, lateral load is taken directly into tension and compression in the beams, allowing for relatively high lateral load transfer. Both the conical shaped ring embodiment and the flat ring embodiment can have a number of variations that provide low bending stiffness but high axial stiffness of the members. The variations include individual rods connecting the inner small diameter ring and the outer large diameter ring. In addition, the manner of material removal to produce the flexible beams can produce a variety of other configurations. These configurations include slots that reverse on themselves to produce multidirectional beams and/or slots that double back on themselves. Additionally, alternating series of drilled holes can leave multi-linked curved beams.
Depending on whether resistance to axial loads, lateral loads, or resistance to combination of both loads is desired, the conical shaped ring embodiments of the attachment and the flat ring may be used alone or in combination. Other variations of the device provide two opposing conical shaped attachments or a conical and flat ring embodiment installed together to provide load capability in both axial and lateral directions while still providing angular flexibility.